Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or an image on paper, film, cloth or the like not only in offices but also at homes. As the inkjet ink, an oily ink, an aqueous ink and a solid-state ink are known but in view of production, handleability, odor, safety and the like, an aqueous ink is predominating.
However, many aqueous inks use a water-soluble dye which dissolves in the molecular state, therefore, despite high transparency and high color density, the aqueous ink is disadvantageous in that the water resistance is low due to water solubility of dye and when printed on so-called plain paper, bleeding is caused to seriously impair the printing quality, and also in that the light fastness is low.
For the purpose of solving these problems, an aqueous ink using a pigment or a disperse dye has been proposed and a large number of techniques thereon are disclosed (see, for example, JP-A-2000-63723 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, these aqueous inks have a problem that the water resistance is, though enhanced to a certain extent, not sufficiently high, the storage stability of the dispersion of pigment or disperse dye in the aqueous ink is poor and clogging readily occurs at the ink ejection port.
Furthermore, when recording is performed on a recording paper having provided on the surface thereof an ink-accepting layer containing a porous inorganic pigment (so-called photographic quality paper), which has been recently created to cope with demand for higher image quality in the inkjet technology, the aqueous ink using a pigment or a disperse dye exhibits poor permeation and the dye or pigment is readily stripped off from the surface by the rubbing with a hand.
For the purpose of solving this problem, a method of incorporating a dye into an acryl polymer, polyurethane or polyester dispersion particle has been proposed (see, for example JP-A-11-286637).
However, these dispersions are disadvantageous in that when a dye is incorporated to a desired concentration, a coloring particle excellent in the dispersion stability can be hardly obtained, and also in that the dye is stripped off, similarly to the aqueous ink using a pigment or disperse dye.
In view of adhesion to the substrate, a UV-curable inkjet ink comprising a polymerizable acrylate, a coloring agent and a UV photo-initiator and being excellent in the adhesion to a plastic substrate is disclosed (see, for example, British Patent No. 2,314,851). However, the coloring agents described in this patent publication all are a pigment dispersion and the color tone of the obtained color image is not satisfied. Furthermore, depending on the polymerizable acrylate selected, a problem arises in the dispersion stability of pigment and disadvantageously, coagulation of ink readily occurs.
When the coloring agent is changed to an oil-soluble dye so as to solve the problem of color tone of the obtained color image, this is found to incur problems that sufficiently high curability is not necessarily obtained due to inhibition of curing polymerization reaction by the oil-soluble dye, the polymerization takes a long time until completion, or huge energy is necessary.